An Enginneer's Choice for the Next Stage of the VPP

I think both replacement offerings from AGL for the next stage are excellent and I would strongly urge you to do your own homework on which to choose. Both are AC coupled systems, but with a difference.

I originally had an SMA 5000TL solar inverter, but that was removed and sold when I installed the DC coupled Sunverge SIS. I have decided to install a Fronius Primo 5 kW solar inverter and a Tesla Powerwall 2 (PW2) battery for the following reasons, not necessarily in order of significance.

1. Tesla claims that the PW2 has a useable energy of 13.5 kWh, as opposed to the 9.3 kWh at 95% discharge for the LG Chem battery. This is a big feature for us because our analysis shows we can benefit significantly from the extra capacity. It is a long way above Sunverge at 8.12 kWh and LG Chem at 9.3 kWh.

2. Tesla are not distributing DC coupled Powerwalls in Australia, but have an elegant solution for prolonged grid outages, which is the next best thing to DC coupling. The PW2 has the ability to sustain a prolonged outage, something Sunverge wouldn’t do as configured for the VPP, nor can the LG Chem/SolarEdge as I understand it. The PW2 is AC coupled, allowing economy of installation, but in the event of a prolonged outage, Tesla tell me that the battery can be discharged to as low as 2% , and still energise the battery inverter at 8 am the next morning whilst remaining safely disconnected from the grid. This enables the solar inverter to see and synchronise to a 240 volt 50 Hz signal, and run if solar energy is available from the panels. If not, the battery inverter tries again on the hour until, either the solar inverter runs, or the battery gets too low. At that point, it manually disconnects and you will need to flick the switch on the side of the unit at a time when you know solar energy is available.

3. Tesla claims that the PW2 can be discharged to 0% for unlimited cycles when used for solar self-consumption purposes, and still guarantee that the battery will have 70% of its capacity in 10 years. Tesla has told me that the battery capacity is in fact in the range of 14.5 to 15.0 kWh, which means it is being discharged to 7% to 10% of its capacity at most. Guaranteeing an initial 13.5 kWh, and using a scale of 0 to 100% makes it a lot easier to understand charge and discharge limits.

4. AGL have assured me they will not be overriding the charge and discharge limits, as they did with the Sunverge system, meaning you get to use the full available capacity of the battery. I am not aware of the limitations that might be placed on the LG Chem/SolarEdge system. For the PW2 these limits can be set via the Tesla app by the customer. Given that Tesla warrants the capacity of the battery and bases that warranty on the battery being charged and discharged between 0 to 100% for unlimited cycles when used for solar self-consumption or backup only purposes, it would seem sensible to do so. The only limitation might be, as indicated above, that a small reserve should possibly be kept in case of a prolonged grid outage.

5. I’m reliably informed that the PW2 inverter has a rapid response time which will help reduce grid imports. Excess grid importing can occur because there is a delay between when a household load is switched on and the inverter starts. The slower the inverter is, the more grid energy will be imported until the inverter can power the load.

6. The Fronius Primo, with a Fronius Smart Meter, will give me detailed data on imported and exported energy, house and battery consumption (albeit combined), and solar generation, all at revenue grade (stated as 1%) accuracy, and on a single web platform. It seems that currently the SolarEdge inverter has significantly better monitoring facilities than the PW2, but since I have to install a new solar inverter, this will make up for that.

7. I’m hoping that the Fronius Primo solar inverter is a true 5.0 kW inverter, and I have been told by installers that it is. I’m also hoping the PW2 battery inverter is a true 5.0 kW inverter, but that remains to be seen.

8. The design contrast between the large, noisy industrial Sunverge cabinet and the elegant PW2 is huge. The PW2 has an excellent ingress rating of IP67, and the LG Chem has a rating of IP55.The PW2 uses electronic transformerless inverters, as does the Fronius Primo, and most modern systems, compared to Sunverge.

9. The PW2 is liquid cooled with two thermostatically controlled pumps, one on the inlet and one on the outlet, and is almost silent at < 40 dBA. The maximum draw of the pumps, according to Tesla, is about 20 watts.

10. The PW2 can be connected to a modem via Ethernet, and in the event of the modem failure, it will automatically switch to GSM cellular mode.

11. The PW2 uses 50 volt DC batteries, whereas the LG Chem batteries have a nominal voltage of 400. This is entirely personal, but I’m more comfortable with lower DC voltages. The higher voltage is probably more efficient given that it matches the voltage from PV strings.

12. The PW2 has an operating temperature range of -20 to 50 degrees centigrade, while the LG Chem is -10 to 45. The low side is not a problem in Adelaide, hopefully, but the high side is.Tesla claims a round-trip efficiency of >90%, but I’ll believe that when I see it six months down the track.

13. There is a fair chance that Tesla will significantly improve their monitoring apps and software, given the momentum the company has, and their attitude.